The present invention relates to an annular core used in a so-called axial air gap motor with a magnetic and structural gap along an axial direction thereof.
An armature with semi-closed slots in a conventional axial air gap motor (to be referred to as an AAG motor hereinafter) has an annular core 1 and an armature winding 2 wound therearound, as shown in FIG. 1. The core 1 used in the armature has T-shaped teeth 3 and slots 4 formed therebetween. Each slot 4 has a narrow upper opening to constitute a semi-closed slot, thereby decreasing slot ripple loss.
In order to manufacture such an annular core 1, an electromagnetic steel sheet is punched to form a profile defining slots 4 and the pressed plate is wound to constitute the core 2, as shown in FIG. 2.
When the core 1 described above is used in a high-speed AAG motor driven with AC power at a frequency high than a commercial frequency or in a variable speed AAG motor driven by a variable voltage variable frequency power source (to be referred to be as a VVVF power source) controlled by pulse width modulation (to be referred to as PWM hereinafter), large audible frequency noise caused by magnetic distortion and vibration of the magnetic steel plate occurs, resulting in inconvenience.